WikiMechanics

Any vision, sight or ocular experience that could loosely be described as greyish is called an achromatic visual sensation. These sensations are described using words like grey, black, white, dark, bright, silvery, taupe, leaden, ecru, ashen, beige, pale etc. The reference experience for achromatic sensation is seeing the Sun. So to make a binary description of an achromatic visual sensation compare it to seeing the Sun. Report the result using one of the following algebraic statements. If the two experiences are not comparable, then say that the sensation is not an achromatic visual sensation and express this as $\delta_{w}=0$. If the sensation is like seeing the Sun, then say that it is whitish. Express this as $\delta_{w}=+1$. If the sensation is not like seeing the Sun, then say that it is blackish and that $\delta_{w}=-1$. The number $\delta_{w}$ is called the whiteness.

Summary

Adjective		Definition
Whiteness	$\delta_{w} \equiv \begin{cases} +1 &\sf{\text{if a visual sensation is whitish}} \\ \; \; 0 &\sf{\text{if a sensation is not greyish }} \\ -1 &\sf{\text{if a visual sensation is blackish }} \end{cases}$	2-2